I presume the OP means 3/8. Drills come in 1/4, 3/8, 1/2, and 3/4 inch sizes, as far as I'm aware.
But, it was answered above - reduced shank bits. OP, something like these:
http://www.amazon.com/b?ie=UTF8&node=340052011 Though, you'll have to pay attention to whether you need the 3/8 hex shank, or if a round shank will do - depends on your drill chuck.
Personally, if I had to drill out larger holes, and if I already had the bits that didn't fit my drill, I would get a larger drill for the increased power and torque, that would handle my bits. You can get a nice corded 1/2" drill for about $50 (or less on sale).
I recently had to drill a 15/16" hole through the side of my house, plus through a lot of joists in the basement. I had both my cordless drill & cordless impact drill right next to me for the job I was working on, and though it took extra time to get one of my corded drills, plus run an extension cord, I never even considered using the cordless drills for the job.